Antonio Isalgue

Metastable effects on martensitic transformation in SMA

The applicability of shape memory alloys (SMA) for dampers in civil engineering requires guaranteed behavior with well controlled or irrelevant changes after several years of inactivity and/or under the action of daily or seasonal temperature effects. The long time analysis of the aging temperature effects on a NiTi alloy shows similar behavior to other SMA but with higher time constants at temperatures near 373 K: 1.9 days at 410 K and near 100 days at 363 K with an activation energy value near 12500 K (RT=104 kJ mol−1). At the present state of the art, the measurements show that the change of transformation temperatures under the action of the external temperature (or ‘room temperature’ change) is close to 15 K. The direct use of the as furnished material can suffer relevant changes of working properties in devices under the action of direct sunlight or high room temperature working conditions.

METASTABLE EFFECTS ON MARTENSITIC TRANSFORMATION IN SMA Part 4. Thermomechanical properties of CuAlBe and NiTi observations for dampers in family houses

AbstractThe behavior of shape memory alloys (SMA) allows their use as a passive smart material. In particular, the existence of a hysteretic cycle in the domain of the elementary coordinates strain-stress-temperature (σ, ε, T) suggests its application for damping in mechanical and/or in civil engineering. We are working in the application of SMA as dampers for earthquakes in small houses as family homes. For dampers installed in the inner porticos of the house, the suggested SMA is the CuAlBe and, eventually, the NiTi. At room temperature the used SMA wires induces forces situated between 2–3kN/wire. The properties related with the damping applications for CuAlBe and NiTi, i.e., the SMA creep and the self-heating will be presented, together with some other minor stress and temperature effects on NiTi modifying the hysteretic behavior.

Interaction of single variant martensitic transformation with small γ type precipitates in CuZnAl

Abstract Thermally- and stress-induced single-interface and single-variant martensitic transformation, in specimens containing a fine distribution of γ precipitates, was studied. The average precipitate size was 7 nm and the density 8 × 1021 prec./m3. The hysteresis width, considerably larger than the one observed for specimens without precipitates, decreases with the number of cycles, from an initial value of 7.2 K to an asymptotic value of about 1.5 K. A simple model shows that the interaction can be explained essentially by considering the deformation due to the shape change upon transformation of the matrix cavities enclosing the precipitates. The energy to create dislocations and to produce the subsequent plastic deformation of the martensite around the precipitates is of the same order of magnitude as the excess of free energy associated to the hysteresis. Nucleation of the β phase inside a martensite plate was observed with a nucleation energy of about 1.3 J/mol.

Shape memory alloys as an effective tool to damp oscillations

The SMA was studied for their macroscopic application in damping for civil engineering. The study is a synthesis and includes an outline of the models required for the SMA simulation and some case studies using the finite element analysis methods. This work is an overview that focuses in the mitigation of the oscillations in structures induced by earthquakes, and for a reduction of the oscillations amplitude in stayed cables under the action of rain, wind or traffic. The analysis needs the required conditions for each application determining the working conditions. The study includes the number of working cycles, the temperature effects and the cooling actions and, for instance, the action of the cycling frequency. The main target relates the appropriateness of the SMA for each purpose, and the suitability of the SMA device is always experimentally guaranteed. Furthermore, the applicability of the obtained results for SMA and the practical behavior of the SMA dampers were studied in international facilities. The paper includes appropriate suggestions for a correct preparation of the SMA dampers. This work outlines the effects of stress and temperature aging in NiTi, describes the particular structural effects between 18R and 6R, introduces a first attempt in the dynamic properties of the CuAlBe single crystals and summarizes some recent suggestions for damping using SMA.

Damping in civil engineering using SMA. The fatigue behavior and stability of CuAlBe and NiTi alloys

Two types of application in damping of structures by SMA in Civil Engineering are considered. The first one is related to the reduction of the damage produced by earthquakes. The second one is concerned with the increase of the lifetime of the stayed cables in bridges. The analyses of the experimental conditions required for each application are different: Several years or decades without any activity (excepted the summer-winter room temperature parasitic effects) followed by one or two minutes of oscillations under the earthquake affects, or near 100000 oscillations per day with pauses of several hours or days in the damping of stayed cables in bridges. This article analyzes the fatigue behavior of the CuAlBe alloy (appropriate for earthquakes) and of the NiTi alloy. Measurements of the damping of stayed cables indicate that the oscillation amplitude could be reduced up to one-third by using a NiTi wire as a damper device.

Hysteresis loops in stress induced β-18R martensite transformation in CuZnAl

Abstract A detailed study of the characteristics of the hysteresis loops in stress induced transformations was carried out using a high resolution apparatus. Samples with two different thermal treatments, air cooled and water quenched from 1070 K, were tested. The contribution of the nucleation processes to the hysteretic behaviour was evaluated. It was found that the intrinsic pseudoelasticity, due to the interaction of the martensite with the dislocations, plays a crucial role in the phenomenology of the transformation. This interaction prevents the growth of a unique plate and the transformation proceeds by successive nucleation of new plates. The friction associated to the movement of a single interface was also measured. From the experiments the relevant parameters which determine the hysteretic behaviour were identified. The hysteresis loops were computer simulated on the base of these parameters. The results showed a good agreement with the experimental observations.

Damping in civil engineering using SMA Part 2 – particular properties of NiTi for damping of stayed cables in bridges

Abstract Shape memory alloys (SMAs) show particular properties associated to their martensitic transformation between metastable phases. Their use requires a deep knowledge of the SMA behaviour according to the requirements of the application. In this paper, the following are studied: the required fatigue life of the NiTi SMA that easily overcomes 1 million of oscillations for a complete storm; the suitable thermomechanical treatment to minimise or stabilise the SMA creep; and the temperature effects on the damper and the dynamic phenomena effects modifying the hysteretic energy via the self-heating associated to the release and the absorption of the latent heat were studied. The study includes the effects on SMA of the external summer–winter temperatures. La transformation martensitique entre phases métastables est à l’origine des propriétés particulières des Alliages à Mémoire de Forme (AMF). L’utilisation des AMF nécessite une connaissance approfondie de leur comportement associé aux exigences de l’application. Dans cet article, on étudie: (a) la durée de vie requise de l’AMF NiTi, qui peut facilement avoir à surmonter 1 million d’oscillations lors d’une tempête entière. (b) Le traitement thermomécanique approprié afin de minimiser ou stabiliser le fluage de l’AMF. (c) Les effets de température sur l’amortisseur ainsi que des phénomènes dynamiques modifiant l’hystérésis de l’énergie par l’auto-échauffement associé au dégagement et à l’absorption de la chaleur latente. On étudie également l’effet des températures extérieures de l’été et de l’hiver sur l’AMF.

SMA in Mitigation of Extreme Loads in Civil Engineering: Damping Actions in Stayed Cables

Reliable use of Shape Memory Alloys (SMA) in mitigation of extreme load effects requires a deep study of the SMA behavior according to the necessities of the application. Damping the oscillations induced by one storm of three or four days with a strong wind and/or rain usually requires more than one million of working cycles. A SMA damper is checked in to two realistic cables of ELSA and of IFSTTAR. The measurements establish that the SMA damping device reduces drastically the oscillation amplitude. Technical suggestions for the preparation of the dampers built by several SMA wires of NiTi with 2.46 mm of diameter are included. The choice of appropriate length and the choice of the number of required SMA wires are suggested. Moreover, a suitable simulation by proprietary SMA routine inside ANSYS is included.

SMA (Cu-BASED, NiTi) FOR USE IN DAMPING: THE IMPLICATIONS OF RELIABILITY FOR LONG TIME APPLICATIONS AND AGING BEHAVIOR

Guaranteed behavior of SMA in damping needs deep study of the alloy behavior according to the requirements. Pseudoelastic window, evolution of properties, self-heating and fatigue are the major concerns. The analysis shows that selected alloys can do the work. The application to portico and to cable oscillations in realistic systems shows an excellent damping effect. Useful simulation requires a good phenomenological model. A cubic model is built with improved results at low deformation.

Guaranteed behavior in shape memory alloys : Short- and long-time effects related to temperature and phase coexistence

A phenomenological approach, in the parent phase of Cu-Zn-Al shape memory alloy, establishes a predictable model (or mathematical equations) relating the dependence of Ms with the temperature over a long period of time (i.e. seasonal or yearly room temperature). High-resolution resistance and temperature measurements vs. time are used. The long time Ms tracks the external room temperature via two temperature dependent time constants. In steady state, the changes in Ms approach17 per cent of the ‘room’ temperature change. The detailed analysis shows the puzzling disappearance of the after quench effects.